Many different processes and compositions have been proposed for chemically reducing nitrogen oxide levels in an effluent. These proposals call for adding chemicals, dry or in solution, directly to the effluent and achieve significant NO.sub.x reductions. However, none have been identified which add a number of different chemicals at defined, distinct temperature zones to achieve NO.sub.x reductions of greater than 50%, and preferably greater than 75%, with commercially practical residence times. Moreover, some of the techniques are capable of reducing NO.sub.x only at the expense of creating undesirable levels of other pollutants such as ammonia and/or carbon monoxide.
In U.S. Pat. No. 3,900,554, Lyon discloses reducing nitrogen monoxide (NO) in a combustion effluent by injecting ammonia, specified ammonia precursors or their aqueous solutions into the effluent for mixing with the nitrogen monoxide at a temperature within the range of 1600.degree. F. to 2000.degree. F. Lyon also suggests the use of reducing agents, such as hydrogen or various hydrocarbons, to permit the effective use of ammonia at effluent temperatures as low as 1300.degree. F. Although the patent suggests staged injection of the ammonia composition, there remains no teaching of the efficacy of injecting distinct compositions at different temperature zones to optimize NO.sub.x reduction without producing a substantial amount of other pollutants.
In U.S. Pat. No. 4,208,386 Arand et al. disclose that, for oxygen-rich effluents, the temperature of the effluent should be in the range of 1300.degree. F. to 2000.degree. F. for reducing the nitrogen oxides concentration using urea added dry or in aqueous solution. Alkanoic solvents are said to be reducing agents which, like hydrogen, carbon monoxide, etc., enable the effective operating temperature to be lowered to below 1600.degree. F. Disclosed again is the suggestion to inject in increments, but these incremental injections are of the same urea composition and must all be at positions meeting the same temperature and oxygen concentration conditions. The same holds true for U.S. Pat. No. 4,325,924 to Arand et al.
Although the prior art discloses injection of a composition for reducing nitrogen oxides at a number of spaced positions in, for instance, Bowers, in copending and commonly assigned U.S. Pat. No. 4,751,065, and Bowers, in copending and commonly assigned U.S. Pat. No. 4,719,092, each disclosure is related to the injection of the same composition at locations in which the same conditions, such as temperature and oxygen concentration, exist.
Furthermore, although the reduction of the concentration of nitrogen oxides in an effluent to as great an extent as possible is highly desirable, prior art systems for reducing NO.sub.x concentrations are limited, not only by the amount of NO.sub.x reduction that can be achieved utilizing them, but also by the amount of other pollutants, such as ammonia or carbon monoxide, generated as byproducts of the NO.sub.x -reducing process.
What is desired, therefore, is a process for substantially reducing the concentration of nitrogen oxides in an effluent while maintaining a suitably low level of other pollutants.